The relentless ridge and its effect on 99L

When meteorologists looked at the medium range forecasts on the first of August, one thing was clear, it appeared: Troughiness would become more likely east of the Mississippi River by the tail end of the month. Brought on by a change in tropical forcing and an overall adjustment in the hemispheric wave pattern, this seemed like a moderate to high confidence forecast. Those ideas were wrong.

Instead, poor modeling and a changed pattern evolution has led to an entirely different outcome: The mid level ridging, which has become a staple of the East Coast weather pattern since early July, will return. Forecast model guidance has altered the hemispheric pattern evolution to support a -PNA (Pacific North American oscillation), or troughiness on the West Coast of the United States. This will only feed the development of a strong ridge from the Southeast United States into the Mid Atlantic.

Models have trended stronger and more expansive with a ridge on the East Coast by next week.

Models have trended stronger and more expansive with a ridge on the East Coast by next week.

In the immediate Northeast US, the effects of this ridge aren’t too dissimilar to what we’ve been experiencing. Warmer days, some of them hot, with intermittent precipitation on days when frontal boundaries attempt to surge toward our area. The majority of this precipitation, still, will stay well to the north and west over Southeast Canada. Par for this summers course, for the most part.

The aforementioned ridge becomes a much more important piece, however, when discussing its effects on the  (possible) development of Hermine, from its currently named “99L”, or in other words “We should watch this closely” name tag from the National Hurricane Center. The development of said ridge, to the north of the tropical system, will have major impacts on the steering flow and overall “highway” for the tropical system to track.

So, while confidence in the exact location of the storm system remains low, we can still analyze the overall weather pattern and its major pieces to get an idea as to where the storm may be headed. With the tropical system developing northwestward from the Antilles to a position north of Hispaniola, the mid level steering winds give the system a limited envelope of tracks. The storm system cannot track northward toward this ridge; instead it is steered westward away from it toward a weakeness in the height field.

Forecast tracks for 99L from the afternoon models on Wednesday morning.

Forecast tracks for 99L from the afternoon models on Wednesday morning.

Forecast models and ensembles have, unsurprisingly, caught on to this idea. A general northwest heading toward Florida is favored by many forecast models over the next few days, and an eventual re-emergence of the system into the Gulf of Mexico cannot be ruled out either. The intensity of the system, however, remains a considerable uncertainty and will depend greatly on the exact track of the storm. How much influence will 99L feel from the mountains and hills of Hispaniola? How much wind shear will the storm face, and how impactful (if at all) will surrounding storm systems be on its development?

For now, these are all questions that remain unanswered — and a critical piece in understanding the development and eventual evolution of the system. Over the next few days, forecasters will be able to answer some of these questions which will inevitably lead to greater certainty and confidence in the storms evolution — and eventual impacts on the Southeast, Gulf, and entire Eastern Untied States.